Owing to the simplicity of its kidney and the availability of mutants affecting kidney development, the zebrafish pronephros serves as a useful genetic model of kidney development. A set of genes identified in the zebrafish pronephric cyst mutants fleer, elipsa, and sinus affect the development of both the kidney epithelia and the retinal photoreceptors. Defects in epithelial cell function result in cyst formation, suggesting parallels to human kidney cystic disease and, in particular, to a human syndrome called renal-retinal dysplasia. This syndrome describes a pleiotropic genetic defect which gives rise to renal cysts and retinal degeneration (retinitis pigmentosa). Renal cystic disease is of the juvenile nephronophthisis/medullary cystic disease type. Currently, little is known about the molecular basis of these genetic defects. Nephrocystin, the gene responsible for human type 1 juvenile nephronophthisis, has been cloned and shown in vitro to participate in focal adhesion structure or signaling via interactions with other focal adhesion proteins p13OCAS and the tyrosine protein kinase PYK2. In vivo functional analysis of nephrocystin and other interacting proteins will be necessary to fully understand the processes underlying terminal epithelial cell differentiation and the pathogenesis of kidney cystic disease. We plan to use the zebrafish as a genetic and embryological model system to 1) Clone the fleer gene and examine the cellular mechanisms of both cyst formation and retinal degeneration and 2) determine the cellular function of the zebrafish nephrocystin homolog (human NPHP1) and p13OCAS using antisense morpholino oligonucleotides and 3) generate tools for tests of gene function in the kidney by creating new transgenic zebrafish lines using pronephric specific promoters. A better understanding of the genetic and cellular factors that lead to terminal epithelial cell differentiation and function will aid in predicting clinical outcomes and in identifying novel targets for therapeutic approaches to kidney cystic disease.